This invention relates to method and apparatus for removing pollutants from gaseous emissions emanating from aluminum production processes.
One well-known type of aluminum producing method is a reduction electrolytic process, commonly referred to as reduction potlines, wherein aluminum metal is produced from Alumina (Al.sub.2 O.sub.3). Those skilled in the art appreciate that two types of electrodes are used in aluminum reduction potline processes. These included prebaked anodes and Soderberg or self-baking electrodes. In both of these aluminum reduction processes, a number of pollutant substances are generated, such as hydrogen fluoride and other fluorine compounds, carbon monoxide, carbon dioxide, sulphur dioxide and hydrocarbons. The hydrocarbons are normally found in the gas stream evolved from Soderberg-type potlines, whereas in prebaked anode-type plants, the hydrocarbons are evolved in the anode fabrication and baking processes called carbon plants, where such electrodes are fabricated.
The gases evolved in these aluminum plant processes not only contain these pollutants but entrained finely divided solid particles of alumina, carbon and electrolytic "bath" (Cryolite, aluminum and/or calcium fluoride). These emissions are commonly vented by means of specially designed hoods and duct work to pollution control devices, to prevent the discharge of pollutants into the atmosphere and for the recovery of valuable fluorine and alumina contained in the gases. In one well-known pollution control system, metal grade alumina is employed for the adsorption and removal of hydrogen fluoride and other fluorine compounds as well as the entrapment of hydrocarbons from gases evolved in electrolytic aluminum manufacturing processes. A principal advantage of using alumina for pollution control and material recovery is that the alumina along with the recovered fluoride can readily be returned to the reduction pots or cells, for use in the aluminum reduction process. As a result, a substantial fluoride saving can be realized. While these gas cleaning methods, usually referred to as dry scrubbing, have certain advantages, they do involve a relatively high cost, a high level of energy consumption and inefficient or unreliable service.
In certain prior art fume treatment systems, alumina in bulk form is discharged directly into horizontal or vertical ducts, through which the waste gases are conducted. In this type of apparatus, the alumina is quickly airborne in the direction concurrent with the gas flow, resulting in limited contact between the alumina particles and the gases and correspondingly limited adsorption of pollutants by the alumina. This results in a reduced scrubbing efficiency. To resolve this problem, other systems such as that shown in U.S. Pat. No. 3,780,497 induce turbulence into the gas flow to improve mixing of the alumina and the gases to thereby obtain higher scrubbing efficiency. However, the increased turbulent flow combined with the highly abrasive properties of alumina, result in the abrasion of the metallic portions of the apparatus, thus introducing impurities such as iron into the aluminum product. Additionally, the action between alumina particles in the turbulent flow and with the metallic surface of the apparatus results in a breakdown or reduction in alumina particle size. This problem is commonly referred to as alumina degradation. In practice, it has been found that the efficiency of modern aluminum reduction cells is impaired by degraded alumina, particularly since the finer alumina particles inertially segregate from the larger particles in storage and conveying systems which handle the alumina from the fume treatment facilities to the potline reduction facilities.